Chapter 4. Forces and Newton s Laws of Motion. continued

Transcription

1 Chapter 4 Forces and Newton s Laws of Motion continued

2 Quiz 3

3 4.7 The Gravitational Force Newton s Law of Universal Gravitation Every particle in the universe exerts an attractive force on every other particle. A particle is a piece of matter, small enough in size to be regarded as a mathematical point. The force that each exerts on the other is directed along the line joining the particles.

4 4.7 The Gravitational Force For two particles that have masses m 1 and m 2 and are separated by a distance r, the force has a magnitude given by the same magnitude of force acts on each mass, no matter what the values of the masses.

5 4.7 The Gravitational Force

6 4.7 The Gravitational Force

7 4.7 The Gravitational Force Definition of Weight The weight of an object on or above the earth is the gravitational force that the earth exerts on the object. The weight always acts downwards, toward the center of the earth. On or above another astronomical body, the weight is the gravitational force exerted on the object by that body. SI Unit of Weight: newton (N)

8 4.7 The Gravitational Force WEIGHT is a force vector Relation Between Mass and Weight mm W = G E W = mg, r 2 WEIGHT(magnitude) of mass m Your WEIGHT, downward g = G M E r 2 WEIGHT DEFINITION Your weight is the force that gravity applies on your body. W r distance to center of the earth

9 4.7 The Gravitational Force Near the earth s surface r = R E = m Radius of the earth g = G M E R E 2 ( ( ) kg) ( m) 2 = N m 2 kg 2 = 9.80 m s 2 This is why acceleration due to gravity is this value on the earth. Your WEIGHT on the earth W = mg for example: m = 80.0 kg, W = mg = 784 N

10 4.7 The Gravitational Force Near the earth s surface In orbit at altitude = 200 km r = R E 200 km g = 9.80 m s 2 At radius of the earth g = 9.20 m s 2 At 200 km above the earth r = R E km = m = m g = GM E = 9.20 m/s 2 r 2 In low-earth orbit, your weight is almost the same as on earth. NOT ZERO!

11 4.7 The Gravitational Force Can you feel gravity (the gravitational force)? Most people would say yes! Consider standing on the concrete floor. Gravity pulls down on you and compresses your body. You feel most of the compression in your legs, because most of your body mass is above them. Consider hanging by your hands from a 100 m high diving board. Gravity pull down on you and stretches your body.you feel most stretching in your arms, because most body mass is below them. Let go of the 100 m high diving board. While gravity accelerates you downward, what do you feel? You don t feel stretched, and you don t feel compressed. You feel weightless, yes, but your weight is still W = mg.

12 4.7 The Gravitational Force The ONLY thing a person can feel is a stretch or compression of your body parts, mostly at a point of contact. If your body is not stretched or compressed, you will feel like you are floating. Gravity ALONE will not stretch or compress your body. Hanging from the board, the board also pulls up on your arms. Newton s 3 rd law! Standing on the ground, the ground also pushes up on the bottom of your feet. Newton s 3 rd law! While falling, the earth pulls on you and you pull on the earth. Gravity requires no contact. YOU CANNOT FEEL GRAVITY.

13 4.8 The Normal Force Definition of the Normal Force The normal force is one component of the force that a surface exerts on an object with which it is in contact namely, the component that is perpendicular to the surface. F sometimes written as F N F W

14 4.8 The Normal Force F Net = F + W + F H = 0 = F + ( 11 N) + ( 15 N) F : Normal (or perpendicular) force F is the magnitude F H = 11N F = +26N F = +26 N F Net = F + F H + W = 0 = F +11 N+ ( 15 N) F = +4 N F H = 11N W = 15N W = 15N F = +4N

15 4.8 The Normal Force Apparent Weight = Normal force acting on an object The apparent weight of an object is the reading of the scale. It is equal to the normal force the scale exerts on the man. Also, by Newton s 3 rd law It is equal to the normal force the man exerts on the scale. a = g a a 700 N 1000 N 400 N 0 N Acceleration a = 0, v constant Acceleration a, upward Acceleration a, downward Free fall a = g, downward

16 4.8 The Normal Force For the person being accelerated (a) F y = F + W ; F y F + W = ma = ma F = F, upward +F + ( mg) = ma F = mg + ma W = mg, downward apparent weight true weight a upward: apparent weight > true weight a downward: apparent weight < true weight

17 4.9 Static and Kinetic Frictional Forces When an object is in contact with a surface forces can act on the objects. The component of this force acting on each object that is parallel to the surface is called the frictional force.

18 4.9 Static and Kinetic Frictional Forces When the two surfaces are not sliding (at rest) across one another the friction is called static friction. Block is at rest. Net force action on block F R = rope force F R F R F = F R + f S = 0 +F R + ( f S ) = 0 (directions are opposite) F R = f S (magnitudes the same) The harder the person pulls on the rope the larger the static frictional force becomes. F R Until the static frictional force f S reaches its maximum value, f S Max, and the block begins to slide.

19 4.9 Static and Kinetic Frictional Forces The magnitude of the static frictional force can have any value Max from zero up to a maximum value, f S µ S Friction equations are for MAGNITUDES only. f S f S Max (object remains at rest) f S MaX = µ S F, 0 < µ S < 1, coefficient of static friction. m f S W 20 kg With no other vertical forces, F = W = mg F F R normal force of table on the mass

20 4.9 Static and Kinetic Frictional Forces Note that the magnitude of the frictional force does not depend on the contact area of the surfaces.

21 4.9 Static and Kinetic Frictional Forces Static friction opposes the impending relative motion between two objects. Kinetic friction opposes the relative sliding motion motions that actually does occur. is called the coefficient of kinetic friction.

22 4.9 Static and Kinetic Frictional Forces

23 4.9 Static and Kinetic Frictional Forces The sled comes to a halt because the kinetic frictional force opposes its motion and causes the sled to slow down.

24 4.9 Static and Kinetic Frictional Forces Suppose the coefficient of kinetic friction is 0.05 and the total mass is 40kg. What is the kinetic frictional force?

25 4.10 The Tension Force Cables and ropes transmit forces through tension.

26 4.10 The Tension Force A massless rope will transmit tension undiminished from one end to the other. If the rope passes around a massless, frictionless pulley, the tension will be transmitted to the other end of the rope undiminished.

27 4.11 Equilibrium Application of Newton s Laws of Motion Definition of Equilibrium An object is in equilibrium when it has zero acceleration.

28 4.11 Equilibrium Application of Newton s Laws of Motion Reasoning Strategy Select an object(s) to which the equations of equilibrium are to be applied. Draw a free-body diagram for each object chosen above. Include only forces acting on the object, not forces the object exerts on its environment. Choose a set of x, y axes for each object and resolve all forces in the free-body diagram into components that point along these axes. Apply the equations and solve for the unknown quantities.

29 4.11 Equilibrium Application of Newton s Laws of Motion

30 4.11 Equilibrium Application of Newton s Laws of Motion

31 4.11 Equilibrium Application of Newton s Laws of Motion Force x component y component

32 4.11 Equilibrium Application of Newton s Laws of Motion The first equation gives Substitution into the second gives

33 4.11 Equilibrium Application of Newton s Laws of Motion

34 4.12 Nonequilibrium Application of Newton s Laws of Motion When an object is accelerating, it is not in equilibrium.

35 4.12 Nonequilibrium Application of Newton s Laws of Motion The acceleration is along the x axis so

36 4.12 Nonequilibrium Application of Newton s Laws of Motion Force x component y component

37 4.12 Nonequilibrium Application of Newton s Laws of Motion

38 4.12 Nonequilibrium Application of Newton s Laws of Motion